Contrast control in electrostatic copying utilizing liquid development

ABSTRACT

CONTRAST CONTROL IN AN ELECTROSTATIC COPYING PROCESS EMPLOYING POLAR LIQUID DEVELOPMENT IS PROVIDED BY CONTROLLING THE EXPOSURE OF AN IMAGE RETENTION SURFACE IN ORDER TO ESTABLISH CHARGED INFORMATION AREAS HAVING A TOTAL AREA A&#39;&#39;1, WHICH HAS A PROPORTION RELATIONSHIP R1 TO SURROUNDING BUFFER AREAS A&#39;&#39;B. THIS RATIO R1 DIFFERS FROM A PROPORTIONAL RELATIONSHIP RO EXISTING BETWEEN INFORMATION ELEMENT AREAS A1 AND BUFFER AREAS AB OF THE OBJECTIVE.

June 18, 1914 .L. WHITTAKER ,7

CONTRAST c ROL IN ELECTROSTATIC COPYING UTILIZING LIQUID DEVELOPMENT Filed Jan. 28, 1972 2 Sheets-Sheet 1 2 Sheets-Sheet z G. L. WHITTAKER CONTRAST CONTROL IN ELECTROSTATIC COPYING UTILIZING LIQUID DEVELOPMENT Wk 33 xiwkwa All ERREQQ $5 2 SfONDS 3 sac/v05 4 s'cozvos o sEcazvD /.2 [4 A6 OK/G/AWL ozlvs/rr (05.1567) MEEA i556 5 5km any 7.

I I I II- 20 8642 June 18, 1974 Filed Jan. 28, 1972 A mum: \CDQEG EEK Wk g Al x R556 LEVEL l I i l l l l l United States Patent 01 lice 3,817,748 Patented June 18, 1974 3,817,748 CONTRAST CONTROL IN ELECTROSTATIC COPYING UTILIZING LIQUID DEVELOPMENT Gary L. Whittaker, Penfield, N.Y., assignor to Xerox Corporation, Rochester, NY. Filed Ian. 28, 1972, Ser. No. 221,669 Int. Cl. G03g 13/10 U.S. Cl. 96-1 R 11 Claims ABSTRACT OF THE DISCLOSURE Contrast control in an electrostatic copying process employing polar liquid development is provided by controlling the exposure of an image retention surface in order to establish charged information areas having a total area A',, which has a proportion relationship R, to surrounding buifer areas A' This ratio R, differs from a proportional relationship R existing between information element areas A, and buffer areas A of the objective.

This invention relates to electrostatic copying. The invention relates more particularly to improvements in electrostatic copying employing liquid development techniques.

In a known electrostatic copying process, a latent image is established electrostatically on an image retention surface and is developed by contacting the surface with a liquid developing material. The development material ad heres to the surface and conforms to the image. The image is then transferred to a record medium for recording.

In one image development process sometimes termed polar liquid development, the liquid developer or ink is delivered to the image retention surface by a dispensing member. The ink transfers from the dispensing member in image configuration to the retention surface by virtue of electrostatic forces established by charged areas on the image retention surface. This polar liquid development process is described in detail in U.S. Pat. 3,084,043 which is assigned to the assignee of this invention.

It is often desirable to reproduce with electrostatic copying techniques an image which exists in halftone format. As is well known, a halftone printing format utilizes information elements such as dots or the like. Contrast between different subjects in an image is established in halftone format by the difference in density or relative size of dots in one area with respect to those in another. The density, D, of a halftone image can be expressed as:

-F-=fraction of the area covered by the dots, and R=reflectivity of the ink dots relative to the background paper.

If R is much less than one as is normally true for densely colored inks and if in addition F is much less than one, then Thus, for relatively dense inks the density is approximately proportional to the fractional area covered by the dots when this area of coverage is not too large.

At times it is desirable to provide a control of the contrast or gamma between a halftone original and copy in an electrostatic process employing liquid development. Gamma is defined as the slope of the curve of print density versus original density. The gamma of the imaging process with the above considerations in mind then becomes approximately equal to the ratio of the dot area in the image to the dot area in the halftone original. It would be further beneficial to provide a contrast variation wherein informational elements are varied in size within a halftone copy which is reproduced from a continuous tone original in an electrostatic copying apparatus employing liquid development.

Accordingly it is an object of this invention to provide an improved electrostatic copying process.

Another object of this invention is to provide an improved electrostatic copying process employing liquid development.

Another object of the invention is to provide contrast control in an electrostatic copying process employing liquid development.

Another object of the invention is to provide an electrostatic copying process for halftone originals wherein a halftone informational element is varied in size.

A further object of the invention is to provide a process for the production of a halftone copy from a continuous tone original.

As indicated hereinbefore the contrast density between a halftone original and a halftone image is approximately equal to the ratio between the dot area in the image to the dot area in the halftone original. I have found that in an electrostatic copying process employing liquid development, the liquid developer which is transferred to the electrostatically charged image retention surface will flow laterally from a highly charged information elemental area to surrounding areas in proportion to residual charge intensity existing in the surrounding area. Control of the area occupied by the liquid developer dots on the image retention surface will then be a function of the residual charge intensity in the area surrounding an information elemental area.

In accordance with the general features of this invention, contrast control in an electrostatic copying process employing polar liquid development is effected by controlling the exposure of an image retention surface in order to establish charged information areas having a total area A',, which has a proportional relationship R, to surrounding buffer areas A' This ratio R, differs from a proportional relationship R existing between information element areas A and butfer area A of the object. In accordance with a more particular feature of the method of this invention, an electrostatic copying process for increasing contrast comprises the steps of forming a uniform electrostatic charge on an image retention surface; altering the charge on the image retention surface in order to provide a large plurality of highly charged information elemental areas Ag, surrounded by buffer areas A' bearing a relatively high residual charge to which ink will be laterally drawn and arranged in an image configuration corresponding to a halftone original; delivering a developing liquid to the retention surface for developing said image; and transferring the developed liquid image to a record medium.

These and other objects and features of the method of this invention will become apparent with reference to the following specifications and to the drawings wherein:

FIG. 1 is a schematic view of an electrostatic copying apparatus employing liquid development and operating in accordance with the method of this invention;

FIG. 2 is a side view of a liquid developing material transfer member of FIG. 1;

FIG. 3 is a sectional view of an image retention surface and liquid developing material transfer member illustrating transfer of a liquid developing material to the image retention surface;

FIG. 4 is a diagram illustrating the lateral flow of liquid developing material and the associated surface charge on an image retention surface for one value of residual charge retention;

FIG. 5 is a diagram illustrating the lateral flow of liquid developer material and the associated surface charge on an image retention surface for a second value of residual charge retention:

FIG. 6 is a chart illustrating the variation in Gamma for different exposure times; and

FIG. 7 is a characteristic curve of image density versus logarithm of exposure for a halftone reproduction.

Referring now to FIG. 1, an electrostatic copying apparatus employing liquid development includes a rotatably mounted drum 8 having an image retention surface comprising, for example, a layer 10 of vitreous or amorphous selenium positioned on an outer surface 11 of the drum. The drum continuously rotates in a counterclockwise direction past a charging electrode 12 which establishes a uniform electrostatic charge on the image retention surface. The charging electrode 12 comprises, for example, a high voltage corona discharge electrode adapted to supply ions or electric charges to the image retention surface. The uniformly charged image retention surface is rotated to an imaging station 13. There is positioned at the imaging station a means for projecting an image on the charged surface for forming a latent electrostatic image conforming to the original image. This means includes a projection system including a lamp 14, a photographic transparency 15 hearing an image which is to be reproduced, and a lens 16 for focussing the image on the uniformly charge surface 10. The latent electrostatic image which is thus formed is rotated by the drum to a development station referenced generally as 11. At the development station E, which is described in greater detail hereinafter, the latent electrostatic image is developed and is then transported by the drum to an image transfer station 1 8 at which location the image is transferred from the drum to a suitable transfer web 20. The web 20 comprises, for example, a web of liquid developer receptive paper or the like which is fed from a supply roll 22 to a take-up roll 24 and passes between the drum 8 and a transfer roller 26. After image transfer, the image retention surface is rotated past a cleaning station designated generally as g; where residual developer material, if any, is removed by a rotating brush from the drum surface in order to prepare the drum for recycling through the above described operational stations.

A developer means is positioned at the development station E for providing liquid development of the latent electrostatic image on the image retention surface 10. The developer means includes a liquid developer dispensing member comprising a rotating drum 30, an inking roller 32 and a reservoir 34 containing developer ink 36. The drum which is described in greater detail hereinafter has deposited on its surface at the inking station a film of ink for transport to the rotating drum 8 and for application to the image retention surface 10. The development ink 36 is withdrawn from the reservoir 34 by the inking roller 32 which is partially submerged in the ink and is conveyed to the drum 30. A doctor blade 38 is mounted on a support 40 and is positioned for bearing against the developer dispensing drum 30 in order to remove excess ink from the dispensing drum 30 after loading and prior to application to the image retention surface.

The surface of the developer dispensing drum 30 of FIG. 1 is shown in greater detail in FIGS. 2 and 3. The drum 30 is shown to comprise a metal base having formed on a surface thereof a raised pattern 52 formed, for example, by a plurality of fine raised lines, dots or other geometric configuration. A triangular-helix-patterned gravure applicator drum has been found particularly advantageous in that a two walled structure of a gravure cell of this configuration will yield less overall image background with respect to other wall to wall gravure cell configurations and any tendency for air entrapment in the individual gravure cells is substantially diminished during the inking operation. A particular surface which has been found to be advantageous includes to 200 cells or grooves per linear inch. The pitch is on the order of 5.5 mils and the cell depth is on the order of about 4 mils. The surface of drum 30 is characterized by being wettable by the liquid developer in the depressions or valleys formed between the raised peaks of the raised pattern but substantially non-wettable along the upper surface of the raised lines or peaks. This is accomplished, for example, by the use of a support base 50 having a relatively uniform surface and comprising material that is usually wettable by the liquid developer material employed.

FIG. 3 illustrates the developing drum in contact With the image retention surface 10. As indicated, the image retention surface 10 comprises, for example, a vitreous or amorphous selenium coating 10 which is photosensitive and which is fixed on a metallic drum surface 11. Prior to contact with the developer member 30 the image retention surface 10 bears a latent electrostatic image characterized by a variation in the charge along its surface. This charge pattern, for example, is indicated by the plus signs along its outer surface as viewed in FIG. 3. The surface 10 is brought into contact with the developer dispensing member 30 and the varied lines or dots on the developing dispenser member bear against the surface. Circuit means including a battery 60 and a shunting potentiometer 62 are provided for establishing a developer transfer potential between the image retention surface 10 and the developer member 30. One electrode of the battery 60 is connected to the drum surface 11 while a voltage adjusting arm of the potentiometer is connected to the base 50 of the ink transport member. In those areas where an electric charge exists on the image, it has been found that the liquid developer creeps up the side walls of the raised configuration 52 and into contact with the surface 10. In background areas, little or no charge exists on the surface 10 and transfer of the liquid developer does not occur. Those uncharged areas of the surface 10 therefore remain out of contact with the developer. Thus, the liquid developer reaches the plate only in image charge areas to provide an ink development of the latent electrostatic image. An electrostatic copying system employing liquid development of the type described is disclosed in U.S. Pat. 3,084,043 and the disclosure of that patent is incorporated herein by reference.

As indicated hereinbefore, an object in halftone format comprises a large number of information elements, while preferably formed as circular dots, can be formed in other suitable configurations. Contrast is established between different subjects in an object by providing variations in the size of information elements in the image. Each of the information elements in a positive halftone object, for example, is colored uniformly gray or uniformly black in a monochromatic reproduction system and is separated and distinguishable from adjacent elements by a surrounding buffer area of contrasting white or gray. The buffer areas may alternatively be considered a background field upon which the information elements are formed. In the electrostatographic reproduction of an object in halftone format, a latent electrostatic image is formed on an image retention surface. The latent electrostatic image comprises a corresponding plurality of information elements which are uniformly charged and which conform in number and arrangement to the information elements of the object. The electrostatographic imaging process provides non-imaged areas of relatively high uniform charge on the photoreceptive surface 10 which correspond to the information element areas of the object and have a total area A',,, while the imagead areas which have been partially or fully discharged correspond to the buffer areas of the object. Depending upon the characteristics of the projection system employed with the electrostatographic apparatus, the latent electrostatic information elements and the buffer areas will occupy the same areas as they occupy in the object or will be proportionally enlarged or reduced in size. In any case, the relative areas of the information elements and the surrounding buffer areas in the image are ordinarily proportional to their respective areas in the object. Development of the image by transfer of ink to the image retention surface results in the formation of a large number of inked information elements. Upon transfer to the charged image retention surface, I have found that the development ink initially deposits on charged information elements and will then spread laterally and substantially uniformly along the surface from the information element into a portion of the surrounding buffer area. This lateral flow of ink occurs when a charge, termed the residual charge, having an intensity below a development threshold intensity remains in the incompletely exposed and partially charged buffer area. By development threshold is meant a uniform surface charge density or potential of an information element at or above which potential the ink is transferred directly from the development drum 30 to a charged surface. As indicated, the ink is not transferred directly to the buffer area from the drum 30 but will subsequently transfer laterally into this area A' in accordance with the magnitude of the residual charge in this area after having first been attracted to the highly charged information element area. This lateral transfer characteristic is represented in FIGS. 4 and 5 wherein the threshold voltage, the residual charge level, and the potential for a halftone information element on the image retention surface are indicated. When a relatively high residual charge level exists as is illustrated in FIG. 4, then the ink which is initially transferred to the highly charged information element area A' will spread laterally under the influence of the residual charge thereof from the element area into an area A' which is a portion of the buffer area surrounding the element area. However, when the residual charge level is relatively low, as is illustrated in FIG. 5, the lateral flow force exerted on the ink is also relatively low and the ink remains in the highly charged information element area.

In accordance with the general features of this invention, a method for varying the contrast of an image with respect to an object which is presented in halftone format in an electrostatographic copying apparatus is provided by controlling the exposure of the photoreceptive surface during the imaging step. In accordance with a particular feature of the invention, a method for increasing the contrast of an image with respect to an object which is presented in halftone format in an electrostatographic copying apparatus utilizing polar ink developer is provided by controlling the residual charge level on an image retention surface. The control of the residual charge level establishes the extent of lateral movement of the polar developer ink from the information element area into the surrounding buffer areas. The ratio of the total area A; occupied by the information elements in the object to the area A occupied by the buffer area in the object has a value of R The ratio of the total area A in the image to which ink is drawn to the total area to which ink is not drawn A in the image has a value of R In present day electrostatographic reproduction apparatus for reproducing objects in .halftone format, a proportionality exists between the total areas occupied by the information elements and the buffer area in the object as well as between the information element areas and the buffer areas in the image. While the area occupied by the information and buffer areas in the image may be larger or smaller than the corresponding areas occupied by the information element and buffer areas in the object, the value of the ratios R and R have heretofore been substantially equal. In accordance with this invention, the ratio R is made different than the ratio R during the imaging process. The contrast between the image and the object is accordingly altered. During the formation of a latent electrostatic image in a halftone imaging process, the relatively highly charged information elements are created by uniformly charging the image retention surface and by exposing and discharging charge in the buffer areas. For a particular illumination level I and interval time T the exposure of the image retention surface will provide an image having a substantially same contrast as the object. By decreasing the exposure through reductions in the illumination intensity level or by reducing the time of exposure or by combinations of both, the charge in the buffer areas surrounding the information element areas on the image retention surface will be raised to a level causing the lateral flow of ink from the information element areas into the surrounding areas. The information areas A on the photoreceptive surface will then include the area occupied by the information element A' as well as the inked portion of the bufier area indicated as A The amount of ink flowing into the buffer areas will depend upon the residual charge remaining on the photoconductor surface and can include the entire buffer area associated with an information element when the charge in this area is not materially altered from the level of the charge initially established on a surface to the absence of the flow of ink from the information element area into the buffer area at relatively low levels of residual charge or when the buffer area is fully discharged. Alternatively, the contrast of the image relative to the object can be decreased by providing for the scattering of light into the proportionally imaged information element area at the photoreceptive surface. This can be accomplished by defocusing of the image at the photoreceptive surface. The defocusing will result in the scattering of light into the information element area and will decrease the ratio R Referring once again to FIG. 1, there is illustrated alternative means for varying the exposure of the image reception surface to the object. The lamp 14 which is shown to be an incandescent lamp is energized by current derived from a source of potential shown to be a battery 70. Current from this battery flows to the lamp 14 through a switch 72 and through a current adjusting rheostat 74. For a lamp of fixed electrical energization, the exposure of the surface 10 to the object by closing switch 72 for a predetermined interval of time T will provide a repoduction having a contrast equal to that of the object and the ratios of R and R will be substantially equal. An increase in contrast in the reproduced image with respect to the object is provided by exposing the photoconductor surface for a period of time T which is less than the period of time T The exposure can thus be varied by closing the switch 72 for the interval time T Alternatively, when the switch 72 is maintained closed for a predetermined period of time T then the contrast can be varied by adjusting the illumination of the photoconductive surface for this period of time. The illumination can be altered by adjusting the lamp current through adjustment of the rheostat 74. Rheostat 74 is initially adjusted to provide a lamp current I which in the predetermined interval of time T will provide a reproduction having a contrast equal to the contrast of the object. The rheostat is then adjusted to provide a current I which is generally less in amplitude than the current I and by closing the switch 72 for the predetermined period of time T the image contrast is increased.

As indicated hereinbefore, contrast can be decreased in the reproduced image by non-proportionally reducing the size of the information elements in the image. This can be accomplished by controlling the light whichis ordinarily projected at the surface for forming a proportionally sized image information element. More particularly, the location lens 16 of FIG. 1 is altered with respect to the transparency 15 and the photoconductive surface 10 in order to cause scattering of light into the area which would be occupied by a proportionally related information element. The scattering is accomplished in a substantially symmetrical manner by the lens with the result that the latent information elements thus formed are non-proportionally related in area to the information element areas in the object. In order to simplify the drawings, the various methods for altering the location of the lens 16 with respect to the object 15 and the photoreceptive surface 10 are not illustrated. However, the mechanism for preparing lens adjustment are known in the art.

A desired variation in contrast between the reproduced image and the object can be effected by a machine operator who controls the exposure of the photoreceptive surface. The operator can vary the exposure from that exposure which will provide a reproduction of equal contrast to the exposure which provides a copy of desired contrast. This can be accomplished manually for example, with the apparatus of FIG. 1 by altering the time of exposure or by altering the amplitude of lamp current. The location of the lens 16 can be also altered in order to effect a change in contrast. Alternatively, an automatic contrast control having a means for sensing the contrast of the object and for causing a variation in the illumination, time of exposure or location of the lens 16 can be provided.

Characteristic curves for imaging a halftone original in accordance with the invention are illustrated in FIG. 6. These curves were generated through the use of a halftone original having a 120 screen count, an image retention surface comprising a 15 4 selenium photoreceptor, and employing carbon pigmented mineral oil based ink. An apparatus of the type illustrated in FIG. 3 was employed for exposure and development wherein the development speed was 5 inches per second and a recording sheet comprised Xerox 4024 type paper. The parameters for these characteristic curves is exposure time in seconds. FIG. 6 illustrates a number of gamma curves for exposure of 0.2 through 4 seconds under the recited conditions.

The method of this invention is also advantageously employed for the production of halftone images from continuous tone originals. This is accomplished by placing a uniform screen transparency having the desired configuration for projecting circular or other types of information elements in contact with the continuous tone original. The continuous tone original will then be reproduced as a halftone image. A characteristic curve for a halftone production is derived from the curves of FIG. 6. With a screen of 0.6 density and operating characteristics otherwise the same as described with respect to those employed for generating the curves of FIG. 6, then the print density values can be predicted from the curves corresponding to the respective exposures at 0.6 density units on the original density axis. This curve is illustrated in FIG. 7. Since exposure is the product of intensity and time, the exposure axis can be considered as varying light intensity from the original.

The method of the present invention is also advantageously used in the reproduction of halftone originals where a desired increase or decrease in print contrast over that of the original is desirable. Such a need exists in monochrome reproductions as well as in color reproduction where proper balance of primary colors is achieved by adjusting the dot size when printing each color. The production of halftone images from continuous tone originals is advantageously employed in producing lithographic printing masters, for example.

There has thus been described a relatively simple and noncomplex method for providing a variation in the contrast when producing a halftone copy in an electrostatic copying process employing liquid developer. The method is applicable both to the reproduction of halftone originals and to the production of halftone copies from a continuous tone original for both monochrome and color reproductions.

While I have described and illustrated the particular steps in carrying out my invention, it will be understood that various modifications may be made in the method described therein without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. In an electrostatographic copying apparatus for providing a reproduction in halftone format of an object wherein the object comprises information presented in the form of a large number of information elements which are surrounded by buffer areas of contrasting color, the ratio of the total area A, of the information elements to the total area A of the buffer areas of the object having a value R a method for providing a halftone reproduction having a contrast dilfering from the contrast of the object comprising the steps of:

establishing a uniform electrostatic charge on an electrostatic image retention surface which is light sensitive;

controllably exposing said charged image retention surface to an image of said object to control the residual charge on the image retention surface and forming a latent electrostatic image in halftone format on said surface by establishing charged information areas on said surface corresponding in number to said object information elements and to which a polar ink will be electrostatically drawn surrounded by buffer areas of relatively less charge to which a polar ink is not drawn, the ratio of the total area A of said areas to which a polar ink is drawn to the total area A to which the polar ink is not drawn having a value R; which differs from the value R of said object; and

contacting the image retention surface with a polar developing liquid whereby the ink transfers to the charged information areas, and the control of the residual charge level on the image retention surface establishes the extent of lateral movement of the polar developing liquid from the information element area into the surrounding buffer areas.

2. The method of contrast control of claim 1 wherein said charged information areas are provided with an area A' for establishing a ratio R which is greater than R 3. The method of contrast control of claim 1 wherein said charged information areas are provided with an area A for establishing a ratio R, which is less than R 4. The method of contrast control of claim 2 wherein each of said charged information areas of area A, includes an information element area of relatively high uniform charge intensity having an area A' which is proportionally related to the area of the information elements of said original and to which area a polar ink will be electrostatically attracted when disposed near said surface, and, a surrounding area A' having an electrostatic charge of less intensity than said element area, but of a magnitude for establishing force on said polar ink for causing the lateral flow of said polar ink into said area A' from said information element area A' 5. The method of claim 4 wherein said charged information areas A; are established on said image retention surface by subjecting the surface to activating electromagnetic radiation thereby discharging portions of the uniformly charged surface in image configuration.

6. The method of claim 5 wherein said imaging is provided by exposing said object under light and wherein the ratio R; is made greater than the ratio R by imaging at a predetermined illumination level and for a period of time T which is less than a period of time T which will provide equal ratios.

7. The method of claim 5 wherein said ratio R is made greater than R by imaging said object under light for a predetermined interval of time and at an illumination level which is less than the illumination level which provides equal ratios of R and R 8. The method of claim 3 wherein said image includes information element areas each having an area A, which is not proportionately related to the area of an information element in said object.

9. The method of claim 8 wherein said information element areas are made non-proportionately smaller than the corresponding element areas in said object by altering the exposure of portions of those information element areas which are proportionately related to the areas of the information elements in the object.

10. The method of claim 9 wherein said exposure of said information element areas in said image is altered by deflecting light into said areas.

11. The method of claim 10 wherein light is reflected 10 2,599,542 6/1952 Carlson 96-45 3,472,676 10/1969 Cassiers et a1 96-1 R 10 3,084,043 4/1963 Gundlach 96-1 LY 1,808,743 6/1931 Barkelew 96-27 X 1,947,848 2/1934 Hatherell et al 96-27 X 1,925,547 9/ 1933 Tuttle et al 96-27 X 3,551,313 12/1970 Walsh 96-1 R X 3,251,685 5/1966 Bickmore 96-1 R FOREIGN PATENTS 253,042 5/ 1963 Australia 96-1 R 675,122 12/ 1963 Canada 96-1 LY RONALD H. SMITH, Primary Examiner I. L. GOODROW, Assistant Examiner US. Cl. X.R.

96-1 LY; 117-37 LE 

